WARNING: This document has been automatically Deferred after 12 months of inactivity in its previous Experimental state. Implementation of the protocol described herein is not recommended for production systems. However, exploratory implementations are encouraged to resume the standards process.

Establishing an XMPP session can require a fairly large number of round trips between the initiating entity and the receiving entity. However, in many deployment scenarios it would be helpful to reduce the number of round trips and therefore the time needed to establish a session. This document describes protocol optimizations and best practices to do just that.

In accordance with RFC 6120 [1], before attempting to establish a stream over TCP the initiating entity needs to determine the IP address and port at which to connect, usually by means of DNS lookups as described in Section 3.2 of RFC 6120. Implementations SHOULD cache the results of DNS lookups in order to avoid this step whenever possible. Similar considerations apply to connections established over one of the HTTP bindings, i.e., either BOSH (see BOSH (XEP-0124) [2] and XMPP Over BOSH (XEP-0206) [3]) or WebSocket (see RFC 6455 [4] and RFC 7395 [5]).

XMPP applications SHOULD cache whatever information they can about the peer, especially stream features data and Service Discovery (XEP-0030) [6] information. To facilitate such caching, servers SHOULD include Entity Capabilities (XEP-0115) [7] data in stream features as shown in Section 6.3 of XEP-0115. Note that for maximum benefit the server MUST include all of the stream features it supports in its replies to "disco#info" queries (i.e., not advertise such features only during stream establishment).

XMPP clients SHOULD cache roster information, and servers SHOULD make such caching possible, using Roster Versioning (XEP-0237) [8] as subsequently included in Section 2.1.1 of RFC 6121 [9].

The parties to a stream can proactively send multiple XMPP-related "commands" in a single TCP packet (as one simple example, the receiving entity can send both the response stream header and stream features in a single packet).

The features that the receiving entity supports (e.g., stream features and SASL mechanisms) are stable over time, which means the initiating entity can assume support for certain features and send certain XMPP-related commands without discovering at the time of each connection attempt that the receiving entity supports them.

Together, these assumptions enable the parties to reduce the number of round trips needed to complete the stream negotiation process by "pipelining" XMPP-related commands over the stream.

Note well that pipelining at the XMPP layer is not to be confused with HTTP pipelining, which was added to HTTP in version 1.1 and which is not encouraged when using the HTTP bindings for XMPP.

If both parties support pipelining, they can proceed as follows over the TCP binding (the examples use the XML from Section 9.1 of RFC 6120 for the client-server stream establishment, but the same principles apply to server-to-server streams).

In the client-to-server half of the first exchange, the client assumes that the server supports the XMPP STARTTLS extension so it pipelines its initial stream header, the <starttls/> command, and the TLS ClientHello message.

Now the parties complete the TLS negotiation (i.e., some combination of the TLS messages specified in RFC 5246); for our purposes we don't count these round trips because they are the same no matter whether we use pipelining or not.

At the end of the TLS negotiation, the server knows that the client will need to restart the stream so it proactively attaches its response stream header and stream features in the same TCP packet at the TLS Finished message, thus starting the next exchange.

In response, the client pipelines its initial stream header with the command for initiating the SASL authentication process (including, if appropriate for the SASL mechanism used, the "initial response" data as explained in Section 6.3.10 of RFC 6120).

Without pipelining, the second exchange would require another 2 round trips; with pipelining it requires only 1.

At this point the client and server might exchange multiple SASL-related messages, depending on the SASL mechanism in use. Because this specification does not attempt to reduce the number of round trips involved in the challenge-response sequence, we do not describe these exchanges here.

When the client suspects that it is sending its final SASL response, with pipelining it appends an initial stream header and resource binding request.

The server then informs the client of SASL success (including "additional data with success" as explained in Section 6.3.10 of RFC 6120), sends a response stream header and stream features, and informs the client of successful resource binding.

Without pipelining, this exchange would require another 3 round trips; with pipelining it requires only 1.

Therefore, without pipelining the XMPP exchanges for stream establishment require at least 6 round trips (and perhaps more depending on the SASL mechanism used); with pipelining the minimum number of round trips is 3.

Naturally, for typical client-to-server sessions, additional round trips are needed so that the client can gather service discovery information, retrieve the roster, etc. As noted, these steps can be reduced or eliminated by using entity capabilities and roster versioning.

Note: If the client does not expect to receive a SASL challenge from the server but simply a success or failure notification (e.g., when using a simpler SASL mechanism such as PLAIN RFC 4616 [13]), then it can also pipeline its XMPP resource binding request with the BOSH session creation request.

The BOSH connection manager then returns a session creation response with a pipelined SASL authentication response.

Without pipelining, this exchange would require 2 round trips; with pipelining, it requires only 1.

If the SASL exchange involved a challenge, in its final SASL response the client includes an XMPP resource binding request (note that the BOSH <body/> wrapper includes a restart='true' attribute, instead of sending this in a new empty <body/> as shown in XEP-0206).

An XMPP stream restart is required by RFC 6120 at this point; however, the server can include the stream features element (if any) along with the SASL authentication success and XMPP resource binding success notifications, as shown in the following example.

Without pipelining, this second exchange would require 3 round trips; with pipelining, it requires only 1.

Therefore, without pipelining the XMPP exchanges for stream establishment over BOSH require at least 5 round trips (if the SASL mechanism is not multi-stage, and perhaps more depending on the SASL mechanism used); with pipelining the minimum number of round trips is 1.

Note: It might seem that with pipelining the minimum number of round trips is 2. However, consider the case of a session creation request that includes (a) a SASL authentication request for a SASL mechanism that is not multi-stage, such as PLAIN, (b) an XMPP resource binding request, and (c) a stream restart request; in the "happy path" the session creation response would include (a) stream features, (b) a SASL authentication success notification, and (c) an XMPP resource binding response. This flow is shown in the following example.

The pain of multiple round trips is magnified if the initiating entity needs to reconnect frequently (e.g., because of intermittent network outages). Although XEP-0124 can be used to mitigate the pain, BOSH is not appropriate for all scenarios and is not currently used in others (e.g., server-to-server streams).

To minimize the speed of reconnection, implementations are strongly encouraged to support TLS Session Resumption (RFC 5077 [14]) in addition to the technologies already mentioned.

Reconnection can be further enhanced by using the stream resumption feature defined in Stream Management (XEP-0198) [15]. XEP-0198 does not legislate exactly when it is safe for the server to allow the client to send the <resume/> request. Clearly, sending it before the stream is encrypted would increase the possibility of replay attacks. However, sending it after TLS negotiation (Step 4 above) but before SASL authentication and resource binding (Steps 5 through 8) would enable the client to begin sending stanzas more quickly. It is a matter of server policy whether to advertise the SM feature after TLS negotiation or only after SASL negotiation.

Because pipelining does not skip any channel encryption or authentication steps, but merely packs them into a smaller number of TCP packets or HTTP request/response pairs, it is unlikely that the foregoing quickstart methods introduce security vulnerabilities. However, the server needs to be careful not to send stream features that it would not otherwise send before a security context is established.

Appendix C: Legal Notices

Copyright

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

16. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

17. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.